Revaporization of cryogenic liquids



Dec. 28, 1965 5. B. FARKAS REVAPORIZATION OF CRYOGENIC LIQUIDS Filed May 15, 1964 TO GAS MAE! SEA

WATER FIG.3

1N VENTOR. B. FARKAS GEORGE ATTORNEY United States Patent C) 3,225,552 REVAIORIZATION (H CRYOGENIC LIQUHDS George B. Farkas, Jackson Heights, N.Y., assignor to Hydrocarbon Research, inc, New York, N.Y., a cor= poration of New Jersey Filed May 13, 1964, Ser. No. 366,971 3 Claims. (Cl, 6251) This invention relates to improvements in vaporizing cryogenic liquids of the class of oxygen, nitrogen and methane.

In the process of transporting natural gas for use as a commrecial fuel at distant points, it has been found desirable to liquefy natural gas at relatively high pressure and low temperature at the well head. Natural gas at 259 F. at atmospheric pressure will remain substantially in the liquid condition and then can be transported by ship, it being recognized that liquid methane has approximately 600 times the energy value of gaseous methane in the same volume. At the place of use of the natural gas, it is then necessary to revaporize the liquid to make the gas available for distribution.

The conversion of liquid methane at 259 F. to useful gaseous methane at 30 F. involves matters of heat transfer which must be taken into account. Not only is this a problem of requiring large amounts of energy, and necessarily at a low cost, but the heat must be available in some form that objectionable icing does not occur. Similar problems occur with other liquefied gases such as nitrogen and oxygen.

My invention concerns primarily the solution of the above problem and the economical means for adding heat to cryogenic liquids which will be effective in increasing the temperature to the vaporization point to vaporize the liquid and heat the vapor to a suitable temperature for economical gas distribution. The problem is particularly severe when the vapor is heated only to a temperture under the freezing point of water.

More specifically, my invention relates to an improved means for the vaporization of liquid methane with sea water.

Other objects and advantages of my invention will appear from the following description of a preferred form of embodiment thereof when taken in connection with the attached drawings in which:

FIG. 1 is a schematic view of a liquid methane vaporiztion circuit.

FIG. 2 is a front elevation of a part of a heat exchange element.

FIG. 3 is a transverse cross section on the line 33 of the exchange unit shown in FIG. 2.

FIG. 4 is an enlarged cross section of a heat exchange tube showing the outline of the formed ice.

In an effort to economically vaporize 100 metric tons per hour of liquid methane, largely at a temperature of 259 F. and a small amount of vapor at 240 F., it is apparent that prior experience taught only the use of apparatus that was either extremely expensive, hazardous, or completely unworkable. It has been suggested, for example, that the heat required be abstracted for land based refrigeration. This provided refrigeration requirements far in excess of any known to exist at any port at which the liquid methane could be discharged. It also suggested that part of the methane be burned in an open furnace to give off the heat necessary to vaporize the balance and it was found that the fuel costs then amounted to a substantial percent of the total value of the liquid methane revaporized. The use of water, including sea water in conventional units, proved to be objectionable because of the extremely low temperatures at which vaporization of the liquid methane was 3,225,552 Patented Dec. 28., 1965 accomplished and the freeze up of any liquid that came in contact with surfaces much below F., it being recognized that sea water also freezes in the 030 F. range. The fourth alternative of using one or more secondary refrigerants such as propane, ethylene, etc., involved expensive apparatus and reverse refrigeration circuits which were as costly as the equipment used for liquefying the methane in the first instance.

Unexpectedly I have found that sea water can be an effective heat source which is significant not only because of the great temperature difference over the liquid methane but also due to the fact that the average temperature of sea water in North Atlantic ports is in the order of 50 F. and varies only slightly between winter and summer months. Furthermore, sea water is substantially inexhaustible at most ports and the decrease of its temperature as much as 5 or F. is seldom objectionable either to industry or other natural conditions.

As more particularly shown in FIG. 1, a liquid methane storage tank 19 is indicated as having a deep outlet 12 for the purpose of removing liquid and a shallow outlet 14 to remove any vapor that would tend to collect at the top. The liquid is pumped at 16 through a primary heat exchange coil generally indicated at 18. The vapor is separately passed through a secondary heat exchange coil 29 from which the vapor is compressed at 22 and passed through line 24 with both end products discharging into a gas main indicated at 26.

Each of the coils l8 and will be suitably protected from wind by a closure located over a sump having a typical drain 32. The coils are located above the water level in the sump and heated by sea water in line 34, discharging through the several nozzles 36. This water will be suitably pumped from an available source through the pump 37.

The heat exchangers 18 and 20 are of the s-o-called trombone type more particularly shown in FIG. 2 and consisting of continuous coils which, as shown in FIG. 3, will be in various banks, a, b, and c, or more as desired.

The unique feature of my invention is more clearly demonstrated in FIG. 4 in which the coil 49 has an inner wall surface 40a and an outer wall surface 46b and, after use, may have a coating 42 of ice. It will be apparent that with a suitable metal, such as aluminum, the inner surface 49a at the tube side inlet will approach the temperature of the incoming liquid methane which is approximately 259 F. or of the vaporous methane which may be in the range of 240 F. Under normal circumstances, the outer wall 40b will be the same temperature as the inner wall 459a with the result that after a very short period of operation an ice layer 42 will develop. The arrangement of the heating surface, that is, the centerline to centerline dimension of the tubes, is such that great thicknesses of ice can be accommodated. It is also recognized that as the operation continues the ice layer tends to develop until some point at which the heat exchange between the wall 40a and the methane reaches equilibrium, due to the resistance of the ice to the heat flow, with the heat exchange between the outer surface of the ice 42 and water. The heat flow through the ice then will be at such a slow rate that further ice will not form.

However, at such stage the heat continues to be transferred from the water through the ice into the liquid methane and it is possible to calculate an average heat exchange rate which will result in effective heating of the methane.

It is, of course, to be recognized that the heat exchange is also a function of the surface area with the result that as the ice cylinder 42 increases in diameter it materially increases in surface area thereby automatically balancing out an effective heat exchange rate.

It has been observed that the heat exchange rate and the rate of ice formation as constant water rate are a function of the throughput of the liquid methane, and if the liquid methane volume is reduced, the diameter of the ice cake also reduces. The converse is also true.

Maintaining all other conditions such as gas and water flow rate and tube diameter equal, the ice thickness will gradually decrease toward the Warm end to make the unit ice free at the warm gas outlet.

The formation of an ice cake which is calculated to be in the order of to 12 inches in diameter for a throughput of 100 metric tons per day in a coil of 4 to 8 inches in diameter involves no unusual mechanical problems but does require intermediate mechanical support of the coils.

It will be apparent from the foregoing that the actual 'heat exchange surface is ice coated with a non porous liner which would normally be the heat exchange tube. The heat exchange tube itself does not contact the sea water except at randomly distributed points due to maldistribution of the fluids as at the warm end of the heater.

While I have shown and described a preferred form of embodiment of my invention, I am aware that modifications may be made thereof and I, therefore, desire a broad interpretation of the invention within the scope and spirit of the description herein and the claims appended hereinafter.

I claim:

1. The method of vaporizing a cryogenic liquid selected from the class consisting of liquid oxygen, liquid nitrogen, liquefied natural gas and liquid methane, for utilization in a gas distribution system which comprises passing the cryogenic liquid through a confined path,

heating said path by passing water thereover to form a relatively thick ice cake on said path, and thereafter substantially continuously adding further 'heat at a controlled rate to said cryogenic liquid by passing water over the ice cake without substantial formation of further ice, said ice cake being proportioned in thickness to the heat absorption rate and temperature differential between the cryogenic liquid and water stream.

2. The method of vaporizing a cryogenic liquid as claimed in claim 1 wherein the vapor of the liquid is heated to a temperature under the freezing temperature of Water and then compressed to gas distribution system pressure.

3. In combination with a source of liquid methane and a supply of water, a sinuous heat exchange coil of trombone shape having vertically aligned, horizontally spaced interconnected paths, means to pass the liquid methane through the coil and means to pass the water over the paths of the coil in series to vaporize the liquid while forming an insulating surface of ice on the coils, said spacing of coils being greater than the normal thickness of ice formation thereon.

References Cited by the Examiner UNITED STATES PATENTS Re. 23,958 3/1955 Toulmin 62123 X 2,616,604 11/1952 Folsom 62123 X 2,778,205 1/1957 Berger 6258 2,896,419 7/1959 Thompson 6258 2,964,917 12/1960 Webster 6253 X 3,018,634 1/1962 Gilmore 6252 ROBERT A. OLEARY, Primary Examiner.

LLOYD L. KING, Examiner. 

1. THE METHOD OF VAPORIZING A CYOGENIC LIQUID SELECTED FROM THE CLASS CONSISTING OF LIQUID OXYGEN, LIQUID NITROGEN, LIQUEFIED NATURAL GAS AND LIQUID METHANE, FOR UTILIZATION IN A GAS DISTRIBUTION SYSTEM WHICH COMPRISES PASSING THE CRYOGENIC LIQUID THROUGH A CONFINED PATH, HEATING SAID PATH BY PASSING WATER THEREOVER TO FORM A RELATIVELY THICK ICE CAKE ON SAID PATH, AND THEREAFTER SUBSTANTIALLY CONTINUOUSLY ADDING FURTHER HEAT AT A CONTROLLED RATE TO SAID CRYOGENIC LIQUID BY PASSING WATER OVER THE ICE CAKE WITHOUT SUBSTANTIAL FORMATION OF FURTHER ICE, SAID ICE CAKE BEING PROPORTIONED IN THICKNESS TO THE HEAT ABSORPTION RATE AND TEMPERATURE DIFFERENTIAL BETWEEN THE CRYOGENIC LIQUID AND WATER STREAM. 